A lot of girl MTBers we know who ride, often ride in small sized men's shoes. But that's all wrong as generally, even in small sized men's shoes, they have too much upper material for the (usually) narrower female foot. Shimano has addressed this with a range of women's specific MTB shoes, a range capped by the 121W above.
It's not a bad shoe at all with a coated leather upper and tough nylon mesh panels doing a good job of protecting your feet from damage while keep them sufficiently warm/cool to make them usable all year round. Triple velcro straps keep the shoe snugged up while the mid-sole is a glass fibre reinforced plastic affair and is sufficiently stiff so that, for the masochists, you could race in them. The outsole is grippy and hardwearing enough, but we'd have liked to see twin stud mounts to make slippery pushes on muddy trails much easier. We're a bit miffed there isn't a proper racing shoe for women in the Shimano line with a carbon sole and micro buckle, and we challenge Shimano to sort this out! But until they do, ladies will do well in a pair of 121Ws.
